An implantable medical device, for the delivery of stimulation therapy and/or for diagnostic sensing, may include at least one tissue-penetrating fixation component configured to hold the device at an implant location. FIG. 1 is a schematic diagram that shows potential cardiac implant sites for such a device, for example, within an appendage 102 of a right atrium RA, within a coronary vein CV (via a coronary sinus ostium CSOS), or in proximity to an apex 103 of a right ventricle RV. FIG. 2 is a plan view of an exemplary implantable medical device 200, which includes a tissue-penetrating fixation component formed by a plurality of tine portions 230. FIG. 2 further illustrates device 200 including a hermetically sealed housing 220 that contains control electronics 201 and a power source 203 shown in broken outlines, and which defines a longitudinal axis 2 of the device. Housing 220 may be formed of a medical grade stainless steel or titanium alloy and have an insulative layer formed thereover, for example, parylene, polyimide, or urethane. With further reference to FIG. 2, device 200 includes a pair of electrodes, 261, 262, which may form a bipolar pair for cardiac pacing and sensing; tine portions 230 surround electrode 261 and are configured to penetrate tissue in order to hold electrode 261 in intimate contact with tissue, for example, at one of the aforementioned implantation sites, while securing, or fixating device 200 for chronic implantation at the site. Further description of a suitable construction for device 200 may be found in the co-pending and commonly assigned United States Patent Application having the pre-grant publication number 2012/0172690 A1.
With reference to FIG. 3A, device 200 may be delivered to an implant location via a delivery catheter 300. For example, with reference to FIG. 1, if the target implant site is located in the right atrium RA, coronary vein CV, or right ventricle RV, a distal end 310 of catheter 300 may be maneuvered into the heart through a superior vena cava SVC or an inferior vena cava IVC, according to a transvenous delivery method known in the art. FIG. 3A shows a partial cross-section of distal end 310 of catheter 300, which is formed like a cup to hold and contain device 200 for delivery to the implant site. FIG. 3A illustrates device 200 having been loaded into distal end 310 so that a hook segment 231 of each tine portion 230 is elastically deformed, from a pre-set curvature thereof, to an open position, at which a distal segment 232 of each tine portion 230 extends distally toward an opening 313 of catheter distal end 310. Each tine portion 230 is preferably formed from a superelastic material, such as Nitinol. FIG. 3A further illustrates a deployment element 320 abutting a proximal end of device 200 and extending proximally therefrom, through a lumen of catheter 300, and out from a proximal opening 301 thereof. Element 320 may be moved, per arrow M, by an operator to push device 200, per arrow P, out from opening 313 of distal end 310, for example, when opening 313 has been located by the operator in close proximity to tissue at the target implant site.
FIG. 3B, is an enlarged view of distal segment 232 of one of tine portions 230, wherein a tissue-piercing tip 322, which terminates distal segment 232, has just been pushed out through opening 313 of distal end 310 of catheter 300 and into contact with tissue T. FIG. 3B illustrates distal segment 232 supported by the surrounding wall of distal end 310, in proximity to opening 313, so that the push force of deployment element 320 is effectively transferred through tip 322 to first compress the tissue T, as shown, and then to pierce the tissue T for penetration therein, which is shown in FIGS. 3C-D. FIGS. 3C-D illustrate partial tine penetration and full tine penetration, respectively, as deployment element 320 continues to push device 200 out opening 313. It can be seen that the elastic nature of each tine portion 230, once the constraint of the distal end 310 is withdrawn, allows the corresponding hook segment 231 to relax back toward the pre-set curvature thereof within the tissue. The full penetration of tine portions 230, shown FIG. 3D, is representative of acute fixation of device 200 at the implant site, for example, for the evaluation of device performance (e.g., pacing and sensing via electrodes 261, 262). It should be noted that, at some implant sites, tine portions 230 may, at full penetration, extend back out from tissue T, for example, generally toward distal end 310 of catheter 300.
With further reference to FIG. 3D, a tether 350 is shown looping through an eye feature 205 formed at the proximal end of device 200; tether 350 extends proximally through a lumen of deployment element 320 to a proximal end 351 thereof, outside a proximal end of deployment element 320, which may be seen in FIG. 3A. Thus, if the performance of acutely fixated device 200 is unsatisfactory, the operator may use tether 350 to pull device 200 back into distal end 310, thereby withdrawing tine portions 230 from the tissue, so that device may be moved by delivery catheter 300 to another potential implant site. Alternately, if the acutely fixated device 200 performs satisfactorily, proximal end 351 of tether 350 may be severed to pull tether 350 out from eye feature 205 of device 200, and the fully penetrated tine portions 230 continue to fixate device 200 for chronic implant.
The aforementioned co-pending and commonly assigned U.S. Patent Application '690 discloses suitable embodiments of a fixation component having tine portions similar to tine portions 230, wherein the tine portions exhibit a suitable baseline performance, for example, in terms of a deployment force, an acute retraction force (for repositioning), atraumatic retraction, and acute and chronic fixation forces. Yet, there is still a need for new configurations of tine portions for implantable devices, like device 200, that may further enhance fixation.